Both allergies and a person’s place in the sibling pecking order may influence the risk of developing the blood cancer non-Hodgkin lymphoma, new study findings suggest.

Australian researchers found that people who were either elder siblings or only children had relatively lower risks of developing non-Hodgkin lymphoma, or NHL. A similar protective effect was seen among people with a history of hay fever or food allergies.

The researchers speculate that early immune system development may be behind all of these associations. In brief, early infection - more likely with multiple kids in a family - may set the immune system in such a way that allergies don’t develop; but on the other hand this setting may increase the chances of developing lymphoma.

The study, of nearly 1,400 adults with and without non-Hodgkin lymphoma (NHL), found that those who were a first-born or only child were half as likely to develop the cancer as people who were fourth in their line of siblings.

In all, the researchers report in the Journal of the National Cancer Institute, there was a linear increase in NHL risk, such that second-born children had a lower risk than those who were third-born, who were, in turn, less likely than later-born siblings to develop the disease.

Similarly, men and women who had had hay fever at any age were one-third less likely than those without such nasal allergies to develop NHL, while people with a history of food allergies were 70 percent less likely than the food-allergy-free to be diagnosed with the disease.

NHL encompasses a group of cancers that arise in infection-fighting white blood cells called lymphocytes. Little is known about the cause of these lymphomas, though specific viruses do contribute to certain NHL subtypes seen largely in Africa and Japan.

It’s also known that people with immune deficiencies, such as HIV/AIDS, are at heightened risk of developing the cancer. But whether other forms of “immune dysregulation” are related to NHL risk is unknown, according to the authors of the new study, led by Dr. Andrew E. Grulich of the University of New South Wales in Sydney

To study the question, they looked at whether potential indicators of early immune system development-such as allergies and birth order-were related to NHL risk.

A number of studies have found evidence that limited exposure to infection early in life may make a child more likely to develop allergies. The theory is that this lack of germ exposure delays the normal “switch” that the immune system goes through after birth to become an infection-fighting machine. If the immune system is not given an early push in an infection-fighting direction, it may tend to remain hypersensitive to substances that most people tolerate - which is what gives rise to allergies.

Being an only child, or having otherwise limited exposure to other young children, is one factor that can keep a person sheltered from germs early in life.

So it’s possible, according to Grulich’s team, that elder siblings and only children, as well as people with a history of certain allergies, are partially protected from NHL by virtue of their immune responses.

The type of immune response associated with allergies is known as a Th2-dominant response, as opposed to the Th1-dominant response that targets foreign invaders like viruses. If the Th2-dominant response is indeed what lowers NHL risk, Grulich and his colleagues write, the reason for the benefit is unclear.

Further research, they conclude, is needed to confirm the associations found in this study, and to better understand how the Th2 immune response might be involved in NHL.

SOURCE: Journal of the National Cancer Institute, April 20, 2005.